Many areas of the world do not have adequate fresh water supplies, but they are able to obtain seawater. Seawater can be desalinated using reverse osmosis, among other processes. To desalinate seawater by reverse osmosis (RO), the feed water must be pressurized above the osmotic pressure of the feed water. The feed water becomes concentrated during the process, and its osmotic pressure increases. Typical feed water pressures for seawater reverse osmosis (SWRO) are in the range of 50-70 bar.
Given the high feed water pressures, energy costs (typically in the form of electrical consumption) are the largest component of the operating cost of a SWRO plant. Through various improvements, the amount of energy used per unit of water produced by SWRO has decreased over time. For example, high pressure multi-stage turbine pumps have become more efficient, to about 70% nominal efficiency. Power recovery turbines are now used to recover some of the energy in the concentrated brine flow leaving the RO modules. Recovery rates have been optimized to balance the cost of pre-treating and pumping feed water (which decreases with increased recovery rate) with the cost of producing desalinated water (which increases with increased recovery rate). Despite these improvements, however, energy costs are still a significant portion of the cost of desalinated water.
Energy consumption also interferes with adopting advances in RO membrane technology. Advances in RO membrane technology have included membrane elements that are capable of operating above 70 bar, and with recovery rates of 55% or more. In theory, a higher recovery rate should allow for decreased capital costs and decreased raw feed water flow. Decreasing the flow of raw feed water would in turn produce savings in pre-treatment and feed water pumping, and reduce the environmental damage caused by withdrawing seawater. However, as mentioned above, when the feed water is concentrated its osmotic pressure increases. As recovery rate increases, so does the feed water concentration, osmotic pressure and energy consumption. The key to breaking this cycle is to recovery more of the energy imbedded in the brine leaving the RO modules. The pressure of the brine also increases with osmotic pressure. Accordingly, there is more energy embedded in the brine of a high recovery process. If a greater percentage of this embedded energy can be recovered, there will be a direct reduction in energy consumption, as well as the possibility of further reductions due to an increase in the optimal recovery rate.
Despite incremental improvements over time, turbine based pumps and energy recovery devices are limited in their energy efficiency. Turbine based technologies are used because they are familiar and easy to use to produce constant flow rates and pressures through the SWRO plant. Adopting a different approach, Childs et al. described a piston based pumping and energy recovery system in U.S. Pat. No. 6,017,200, entitled Integrated Pumping and/or Energy Recovery System. This system uses a piston driven by a hydraulic pump to provide pressurized feed water to an RO membrane module. The front face of the piston drives the feed water to the RO module. The back face of the piston receives brine from the RO module. The pressure of the brine acting on the back face of the piston reduces the power required from the hydraulic pump to move the piston.
In the Childs et al. system, “energy recovery” valves admit brine to the back face of the piston on a forward stroke. Additional discharge valves allow the admitted brine to leave the piston on a backward stroke. The energy recovery and discharge valves are controlled by a control unit that also operates the hydraulic pump. The control unit synchronizes the movements of the valves with the movement of the piston. Because the piston reciprocates, it must accelerate and decelerate and therefore inherently produces an uneven rate of flow and pressure of the feed water. However, when a set of pistons are used, their output may be synchronized to produce a fluctuating, but nearly constant, combined out pressure. Although subject to various practical difficulties, the Childs et al. system has the potential to efficiently produce a high pressure flow of feed water.